1. Field of the Invention
The present invention relates to improved processes for extracting nylon from waste materials, including mixed waste materials such as floor coverings, such that degradation of the nylon polymer into lower molecular weight polymers, oligomers, and monomers is decreased. The process involves contacting the waste materials with polar solvents, or mixtures thereof at high pressures.
2. Description of Related Art
Nylon is widely used as a fiber, both in the production of textile cloths, and in the production of floor covering materials, such as broadloom carpet and carpet tile. When used in floor covering materials, the nylon is usually predominantly present as part of one or more layers of a multilayered product. Large quantities of floor covering materials are replaced each year, with the resulting used materials often discarded to landfills. Because of environmental and economic concerns about this waste material, there has been a need for an economical process for recovering and reusing used or discarded floor covering materials. However, because these materials often contain complicated composites or laminates that are difficult to separate into their constituent components, recycling processes for these materials too often result in conglomerated products whose value is considerably lower than that of their individual constituents. In some cases, the value of the product of recycling may be equal to or less than the value of the original starting material. Downcycling of this nature is both environmentally and economically wasteful. Because the downcycled product is not as valuable as the original starting material, or even of the original inputs to the production of the original waste material, economic value is lost. Because the downcycled product does not make the best use of its constituents, additional constituent materials must be produced and used, generating an additional environmental burden made particularly heavy because these constituent materials are often based on petrochemicals.
As a result of these considerations, there is a need in the art for processes that can economically separate and recycle one or more of the components of floor covering materials. In particular, there is a need in the art for processes that allow for the separation and recovery of nylon used in floor covering materials, and in particular, of nylon used in the topcloth or facecloth layers of these floor covering materials, in such a way that the value and usefulness of the nylon is not degraded.
Processes for recovering nylon tend to fall into two distinct categories. In one category are processes where the nylon is deliberately depolymerized into monomer or oligomers thereof, which can then be reused by repolymerization. In general, the goal is to reduce as much of the polymer to monomer as possible, in order to make reuse more predictable. However, processes in this category are disadvantageous because depolymerization is expensive, and can lead to side reactions that decrease the amount of monomer available, reducing efficiency, and requiring additional depolymerization and energy consumption to make up for the loss. In addition, depolymerization of nylon 6,6 through aminolysis results in production of hexamethylenediamine (HDMA), but requires an additional source of adipic acid to obtain nylon 6,6, since adipic acid is not economically recovered from the depolymerization process.
In the other category are processes where the nylon is recovered without depolymerization, so that it can be reused without repolymerization. In general, the goal of such processes is to extract or dissolve the nylon and thus separate it from other components of the floor covering material with minimal polymer breakdown. The nylon is then generally obtained in solid form so that it can be reused. Conventional dissolution or extraction type processes can be difficult, if not impossible, to use without downcycling the value of the nylon. At least part of the reason for this is that some polymer breakdown almost always occurs due to the elevated temperatures necessary to facilitate dissolution or extraction on an economically relevant time scale.
For example, nylon fiber that is to be subjected to a dissolution or extraction type recovery process will have had its molecular weight reduced by the original extrusion process to produce the fiber, due to the high temperatures involved. Molecular weight will be reduced further during the conventional extraction or dissolution process, and as noted before will be even further reduced if the nylon is to be re-extruded back into fiber. This cumulative breakdown will render the molecular weight of the nylon inappropriate for reuse as fibers suitable for flooring applications.
Another disadvantage of both types of nylon recycling processes is that they either use a feedstock with a higher content of non-nylon scrap than is desirable for the process, or they go to great lengths to obtain relatively pure or color-free nylon feedstocks. In the case of composite products, such as floor coverings like carpet and carpet tile, the presence of fillers, backing materials, pigments, adhesives, etc. can complicate the nylon recycling effort, requiring that these materials be physically and/or chemically separated from the nylon prior to any nylon fiber-specific processing.
There is therefore a need in the art for a nylon recycling process that does not require depolymerization, that does not significantly degrade the molecular weight of the nylon polymer recovered, and that is useful to prepare nylon suitable for extrusion into fiber using as a feedstock nylon that has previously been extruded into fiber, and in particular, nylon that has been obtained from floor covering material, without the need to excessively physically separate other components from the feedstock, e.g., by using procedures beyond air elutriation.